The present invention generally relates to optical processing using an actuatable structure and, more particularly, to methods and apparatus that facilitate a variety of optical processing functions using a diffractive actuatable optical processor.
Microelectromechanical systems (MEMS) are being employed in an increasing range of applications. The desirability of employing MEMS arises in part because of the ability to batch fabricate such microscale systems with a variety of highly complex features and functionality. Optical processing is one area where MEMS have been used in an increasing number of applications. In particular, MEMS have been used to modulate the intensity of light.
One method of achieving optical modulation using MEMS is by diffractive optical processing. As is conventionally known, the process of diffraction refers to a change in direction and/or intensity of radiation of a given wavelength after the radiation impinges upon a diffracting element (e.g., a reflective diffraction grating).
The process of diffraction results in a number of xe2x80x9cordersxe2x80x9d of diffracted radiation, wherein each order is diffracted at a particular angle and has a particular intensity, based in part on the wavelength of the radiation and various physical properties of the diffracting element. The mathematical relationships governing the process of diffraction are well-known and may be found in a variety of optics texts, for example.
FIGS. 1A and 1B are side views of a conventional MEMS diffractive processor 5 illustrating principles of MEMS-based optical processing. In FIG. 1A, a grating 10 is illustrated having upper grating elements 12 and lower grating elements 14. The separation between upper grating elements 12 and lower grating elements 14, as measured along the path of incoming beam 16, is equal to one-half of the wavelength of incoming beam 16. Accordingly, grating 10 acts to reflect incoming beam 16 to generate an output beam along the path of incoming beam 16.
In FIG. 1B, grating structure 10 is actuated using any known MEMS method of actuation (e.g., upper grating element 12 is displaced downward by electrostatic actuation) to achieve a desired separation between upper grating elements 12 and lower grating elements 14. In FIG. 1B the separation, as measured along the path of incoming beam 16, is equal to one-quarter of the wavelength of incoming beam 16. Accordingly, grating 10 acts to reflect and diffract incoming beam 16 to form output beams 18, corresponding to a first order 18A and a negative first order 18B of diffraction.
FIG. 1C is a cross-sectional side view of conventional MEMS optical processor 5 taken along lines 3Cxe2x80x943C of FIG. 1A. FIG. 1C illustrates an exemplary one of upper grating elements 12, where the ends of upper grating element 12 are fixed to a frame 13. For efficient operation of a grating (e.g., grating 10 of FIGS. 1A and 1B), upon actuation, the separation between upper grating elements 12 and lower grating elements 14 (visible in FIG. 1B above) should be a known and fixed value to allow controlled diffraction of an incoming beam; however, conventional grating structures have had limited success in achieving controlled diffraction because actuatable upper grating elements 12 are fixed at the ends, and are otherwise free-standing. Accordingly, grating elements 12 only achieve a desired separation at a limited region 15 of grating element 12. As a result, conventional MEMS diffractive optical processors have performance issues related to the loss of efficiency arising from the inability to adequately control the MEMS grating structure.
Some additional performance issues relevant to MEMS structures are speed of operation, range of actuation, and physical size of the spatial light modulator. Conventional spatial light modulators suffer from a variety of shortcomings in connection with at least some of the above performance issues. Thus, needs exist for actuatable optical processors having improved control of actuation of grating elements.
Some aspects of the present invention are directed to actuatable optical processors having improved control of separation of grating elements by using a support structure including an actuation beam to support at least one grating element. Some embodiments of the above aspect of the invention are directed to optical processors for use in optical communications functions. Still other aspects of the present invention are directed to increased functionality of optical communications systems that use actuatable optical processors.
For purposes of the present disclosure, the term xe2x80x9cwavelength bandxe2x80x9d refers to a continuous wavelength spectrum over a particular range of wavelengths (e.g., the optical communications xe2x80x9cCxe2x80x9d band from 1525 to 1570 nanometers, or the xe2x80x9cLxe2x80x9d band from 1570-1610 nanometers). Similarly, the term xe2x80x9csub-bandxe2x80x9d as used herein refers to a fraction of a specified wavelength band, and the term xe2x80x9cchannelxe2x80x9d as used herein refers to a specific relatively narrow sub-band having an optical carrier at a particular wavelength that is modulated to carry information. Accordingly, it should be appreciated that a sub-band (as well as a band) of wavelengths may include one or more channels.
In view of the foregoing, for purposes of the present disclosure, an xe2x80x9coptical signalxe2x80x9d refers to a signal comprising one or more channels designated by optical carriers having wavelengths in a range of from approximately 0.2 micrometers to 20 micrometers (i.e., from the ultraviolet through the infrared regions of the electromagnetic spectrum). Optical signals including optical carriers corresponding to several channels are commonly referred to as wavelength division multiplexed (WDM) signals. The phrase xe2x80x9coptical carrierxe2x80x9d as used herein means any information-bearing light beam, independent of any selected modulation scheme.
In some conventional optical communications applications, optical carriers of two or more channels of a given optical signal may be processed differently based on the wavelengths of the carriers. One example of conventional wavelength-based processing of optical signals is referred to as variable optical attenuation, which relates to a controlled attenuation of optical signals across a particular wavelength band (or sub-band). A device that performs this type of function accordingly is referred to as a xe2x80x9cVariable Optical Attenuator,xe2x80x9d or xe2x80x9cVOA.xe2x80x9d For purposes of the present disclosure, the abbreviation VOA is used to refer either to the variable optical attenuation function or a device that performs such a function. In VOA, typically optical carriers corresponding to channels lying in a particular wavelength band or sub-band are uniformly attenuated.
Another example of conventional wavelength-based processing is referred to as gain-equalization filtration, and a device that performs this type of function accordingly is referred to as a xe2x80x9cGain-Equalization Filter,xe2x80x9d or xe2x80x9cGEF.xe2x80x9d As above, for purposes of the present disclosure, the abbreviation GEF is used to refer either to the gain-equalization filtration function or a device that performs such a function. In GEF, the attenuation of one or more particular optical carriers corresponding to channels of an optical signal within a particular wavelength band or sub-band is controlled, so as to compensate for wavelength-dependent gain variations of an optical amplifier through which the optical signal passes (e.g., an erbium-doped fiber amplifier).
Yet another example of conventional wavelength-based processing is referred to as optical add/drop multiplexing, and a device that performs this type of function accordingly is referred to as an xe2x80x9cOptical Add/Drop Multiplexer,xe2x80x9d or xe2x80x9cOADM.xe2x80x9d As above, for purposes of the present disclosure, the abbreviation OADM is used to refer either to the optical add/drop multiplexing function or a device that performs such a function. In OADM, an optical carrier corresponding to a particular channel of an optical signal is added or removed (dropped) in a controlled manner (also referred to as channel dropping). Often, optical signals processed by an OADM may contain several other channels closely spaced in wavelength with respect to the targeted optical carrier to be added or dropped.
According to some aspects of the invention, a wavelength-division multiplexed (WDM) optical signal (i.e., an optical signal having two or more communication channels) is processed so as to spatially separate optical carriers corresponding to different wavelength bands present in the signal, and the spatially-separated carriers then are individually and selectively diffracted. In particular, according to some aspects of the invention, each wavelength band of the optical signal is capable of being independently and variably diffracted. The different wavelength bands each may contain one or more channels each having an optical carrier.
According to additional aspects of the present invention, an optical signal is spatially separated into different wavelength bands and the wavelength bands are individually and selectively diffracted, and the respective zeroth-orders of the diffracted wavelength bands are spatially combined to produce a single processed optical signal. As conventionally known, with respect to a transmission diffracting element, a xe2x80x9czeroth-orderxe2x80x9d of a diffracted wavelength of radiation refers to diffracted radiation whose direction of propagation is essentially parallel to (and in the same direction as) the radiation impinging on the transmission diffracting element (i.e., the zeroth-order radiation is essentially undeflected by the transmission diffracting element). With respect to an unblazed reflection diffracting element, the zeroth-order refers to diffracted radiation having a diffraction angle that is essentially equal to an angle of incidence of the radiation impinging on the reflection diffracting element, with respect to a normal to a surface of the diffracting element (i.e., the angle of incidence and the angle of diffraction are equal). The zeroth-order diffracted radiation from an unblazed reflection diffracting element also is commonly referred to as a xe2x80x9cspecular reflectionxe2x80x9d (i.e., as if from a plane mirror).
In view of the foregoing, one aspect of the present invention is a method of redirecting light of a wavelength from a main pathway using a diffractive optical element having a plurality of reflective grating elements, and a plurality of actuating beams. Each of the plurality of actuating beams of the diffractive optical element are supported over a substrate and support a corresponding one the grating elements over the substrate to form a corresponding auxiliary gap. The plurality of actuating beams and the plurality of grating elements are configured such that a displacement of at least one of the plurality of actuating beams toward the substrate causes the corresponding one of the reflective grating elements to be displaced toward the substrate. The method comprises directing a beam of light along the main pathway and onto the plurality of grating elements, the beam having at least light of a first wavelength and light of a second wavelength, and positioning at least one of the actuating beams relative to the substrate to cause at least a portion of one of the light of a first wavelength and the light of a second wavelength to be diffracted out of the main pathway.
In some embodiment of the first aspect of the invention, the light of a first wavelength is from a source having a plurality of wavelengths directed along the main pathway. Optionally, the method of the first aspect may further comprise spatially separating the light of a first wavelength and the light of a second wavelength prior to directing the beam of light onto the plurality of grating elements. The light of a first wavelength may be an optical carrier in a DWDM signal, and the least one optical carrier may be dropped. Optionally, the at least one of the optical carriers is assymetrically diffracted. In some embodiments of the first aspect of the invention, the method is used to block optical carriers in an OADM multiplexer system. Alternatively, the method may be used to block an empty channel of the wavelength-division multiplexed signal.
A second aspect of the invention is a method of processing at least one optical carrier of a wavelength-division multiplexed signal using a diffracting optical element, the diffracting optical element having a plurality of reflective grating elements, and a plurality of actuating beams. Each of the plurality of actuating beams of the diffractive optical element are supported over the substrate and support a corresponding one the grating elements over the substrate to form an auxiliary gap. The plurality of actuating beams and the plurality of grating elements are configured such that a displacement of at least one of the plurality of actuating beams toward the substrate causes the corresponding one of the reflective grating elements to be displaced toward the substrate. The method comprises directing the at least one optical carrier along a main pathway, and onto the plurality of grating elements, and positioning at least one of the plurality of actuating beams to modify the optical strength along the main pathway of the at least one optical carrier.
Optionally, the method of the second aspect may further comprise spatially separating the light of a first wavelength and the light of a second wavelength prior to the directing the beam of light onto the plurality of grating elements. The modification may be that the at least one optical carrier is dropped. Optionally, the at least one of the optical carriers is assymetrically diffracted. The method may be used to block optical carriers in an OADM multiplexer system. Alternatively, the method may be used to block an empty channel of the wavelength-division multiplexed signal.
Following below are more detailed descriptions of various concepts related to, and embodiments of, methods and apparatus for wavelength-based optical processing. It should be appreciated that various aspects of the invention as discussed above and outlined further below may be implemented in any of numerous ways, as the invention is not limited to any particular manner of implementation. Examples of specific implementations are provided for illustrative purposes only.